1. Field of Invention
The present invention relates to a ball screw used, for example, in an automobile wheel steering device, and also relates to a wheel steering device having the same.
2. Prior Art
Devices that use ball screws for the steering of wheels of an automobile are known. One such device is an electrically operated power steering device that supplements the steering force on the steering wheel of an automobile by an electric motor.
Described in Japanese Patent Application Laid-Open under No. H09-142315 is an electrically operated power steering device using a ball screw. This electrically operated power steering device is so designed that the thread groove diameter of the nut constituting the ball screw is gradually increased as the nut extends axially outward. Therefore, when the axis of the screw shaft deviates or bends with respect to the nut, distortion of the screw shaft and the nut is prevented, ensuring smooth operation of the ball screw.
Further, described in Japanese Patent Application Laid-Open under No. H11-268658 is an electrically operated power steering device using another ball screw. This electrically operated power steering device is concerned with balls constituting a ball screw, and is designed so that the balls disposed in the axial middle region are lager in diameter than those disposed in the opposite axial ends. Therefore, when a deflection or axis deviation occurs in the screw shaft, the distortion of the screw shaft and the nut is prevented, ensuring smooth operation of the ball screw.
With the arrangements of the prior art described above, the following problems are left. In the construction described in Japanese Patent Application Laid-Open under No. H09-142315, the thread groove of the nut must be machined so that the groove diameter gradually increases as the nut extends axially outward, requiring much time for machining, management of the thread groove diameter after machining not being easy, a fact which forms a cause of interference with an effort toward cost reduction.
On the other hand, in the construction described in Japanese Patent Application Laid-Open under No. H11-268658, two kinds of balls must be installed in the middle and on the opposite sides, so that much time is required in assembly operation and the two kinds of balls must be managed, a fact which forms a cause of interference with an effort toward cost reduction. Further, if a mistake is made in incorporating the two kinds of balls, the prevention of distortion, which is the intended object, cannot be attained, possibly incurring the shortening of life of the ball screw.